Our long-term goal is to define the pathogenesis of Non Alcoholic Fatty Liver Disease (NAFLD) in order to develop treatments for this common obesity-related liver disease, To accomplish this, we've studied ob/lob mice because they develop NAFLD spontaneously, and are naturally obese and insulin resistant (the best clinical predictors for NAFLD in humans), oblob mice have a spontaneous mutation in the ob gene that prevents production of leptin. Thus, leptin deficiency causes NAFLD in this strain, Deficiency of- or resistance to- leptin also causes NAFLD in other animals and in humans, This project is trying to define the mechanisms involved. Our GENERAL HYPOTHESIS is that leptin deficiency alters the function of cells that have competent leptin receptors and changes their production of factors that regulate energy homeostasis and viability in hepatocvtes. Thus, injury to mature hepatocytes (which lack functional leptin receptors) develops as a result of changes that occur in immune cells, neuronal cells and endocrine cells that express functional leptin receptors and are directly regulated by leptin. Our work supports this hypothesis. For example, we showed that leptin deficiency causes Kupffer cell dysfunction and reduces hepatic lymphocyte populations that normally regulate the production of Th-1 (pro-inflammatory, anti-fibrogenic) and Th- 2 (anti-inflammatory, pro-fibrogenic) cytokines in the liver. New preliminary data suggest neuronal factors contribute to this immune dysfunction. Given the general importance of immune mechanisms in the pathogenesis of liver disease, this project witl focus on the role of immunologic factors in the pathogenesis of NAFLD. AIM #1 is to determine how leptin deficiency alters hepatic lymphocyte populations and to determine if Th-1 polarization of cytokine producing cells results. Hypothesis: leptin deficiency -> altered activities of cytokines and neurotransmitters that regulate the viabiliity and differentiation of liver/ymphocytes -> selective depletion of certain lymphocyte populations -->inhibited Th-2 cytokine production -> Th-1 polarization within the fiver. AIM #2 is to assess potential consequences of hepatocyte exposure to excessive Th-1 cytokines, Hypothesis: Th-1 cytokines -> altered activity of cytokine-regulated transcription factors -> mitochondrial uncoupling protein-2 induction -> decreased ATP production impaired hepatocyte pro/iferation and increased hepatocyte necrosis. AIM #3 is to determine the importance of leptin itself for the progression of NAFLD. Hvpothesis: Leptin deficiency -> Th-1/Th-2 cytokine imbalance -> hepatic insulin resistance -> early stages of NAFLD, but cytokine imbalance + decreased sympathetic neurotransmitters -> inhibited progression to cirrhosis. In addition to clarifying the mechanisms for NAFLD pathogenesis, this work will also provide information about general mechanisms that mediate the sevedty of liver injury and fibrosis.